A rare particle decay measured at the LHC may be showing one of the strongest recent hints of physics beyond the Standard Model.
A highly unusual pattern detected in rare B meson decays is giving researchers fresh reason to suspect new physics may be hiding beyond the Standard Model.
New findings from research we are conducting at the Large Hadron Collider (LHC) at CERN in Geneva suggest that scientists may be getting closer to evidence of physics beyond what is currently known.
If verified, these signs could challenge the Standard Model, the theory that has guided particle physics for the past 50 years. The results indicate that certain subatomic particles produced in the LHC may behave in ways that do not match the model’s predictions.
Fundamental particles are the most basic known units of matter. They cannot be broken down into smaller parts. Their interactions are governed by four fundamental forces: gravity, electromagnetism, the weak force, and the strong force.
The LHC is a massive particle accelerator housed in a circular tunnel 27 kilometers long beneath the border between France and Switzerland. Its central purpose is to test the Standard Model and look for places where the theory may fail.
The Standard Model remains the best explanation scientists have for fundamental particles and forces, but it is known to be incomplete. It does not include gravity, and it cannot explain dark matter, the invisible and still undetected form of matter thought to make up about 25% of the universe.
In the LHC, beams of proton particles travelling in opposite directions are made to collide, in a bid to uncover hints of undiscovered physics. The new results come from LHCb, an experiment at the Large Hadron Collider where these collisions are analyzed.
The result comes from studying the decay – a kind of transformation – of sub-atomic particles called B mesons. We investigated how these B mesons decay into other particles, finding that the particular way in which this happens disagrees with the predictions of the Standard Model.
A theory under strain
The Standard Model is built on two of the 20th century’s most transformative advances in physics; quantum mechanics and Einstein’s special relativity.
Physicists can compare measurements made at facilities such as the LHC with predictions based on the Standard Model to rigorously test the theory.
Despite the fact that we know the Standard Model is incomplete, in over 50 years of increasingly rigorous testing, particle physicists are yet to find a crack in the theory. That is, potentially, until now.
Our measurement, published in Physical Review Letters, shows a tension of four standard deviations from the expectations of the Standard Model.
In real-world terms, this means that, after considering the uncertainties from the experimental results and from the theory predictions, there is only a one in 16,000 chance that a random fluctuation in the data this extreme would occur if the Standard Model is correct.
Although this falls short of science’s gold standard – what’s known as five sigma, or five standard deviations (about a one in 1.7 million chance) – the evidence is starting to mount. Adding to this compelling narrative are results from an independent LHC experiment, CMS, that were published earlier in 2025.
Although the CMS results are not as precise as those from LHCb, they agree well, strengthening the case. Our new results have been found in a study of a particular kind of process, known as an electroweak penguin decay.
Rare decays sharpen the test
The term “penguin” refers to a specific type of decay (transformation) of short-lived particles. In this case, we study how the B meson decays into four other subatomic particles—a kaon, a pion, and two muons.
With some imagination, one can visualize the arrangement of the particles involved as looking like a penguin. Crucially, measurements of this decay let us study how one type of fundamental particle, a beauty quark, can transform into another, the strange quark.
This penguin decay is incredibly rare in the Standard Model: for every million B mesons, only one will decay in this manner. We have carefully analyzed the angles and energies at which these particles are produced in the decay, and precisely determined how often the process takes place. We found that our measurements of these quantities disagree with Standard Model predictions.
Precise investigations of decays like this are one of the primary goals of the LHCb experiment, and have been since its inception in 1994. Penguin processes are uniquely sensitive to the effects of potentially very heavy new particles that cannot be created directly at the LHC.
Such particles may still exert a measurable influence on these decays over the small Standard Model contribution. This kind of indirect observation is not new. For example, radioactivity was discovered 80 years before the fundamental particles that are responsible for it (the W bosons) were directly seen.
New data will test the anomaly
Our studies of rare processes let us explore parts of nature that may otherwise only become accessible using particle colliders planned for the 2070s. There is a wide range of potential new theories that can explain our findings. Many contain new particles called “leptoquarks” that unite the two different types of matter: “leptons” and “quarks”.
Other potential theories contain particles that are heavier analogues of those already found in the Standard Model. The new results constrain the form of these models and will direct future searches for them.
Despite our excitement, open theoretical questions remain that prevent us from definitively claiming that physics beyond the Standard Model has been observed. The most serious question arises from so-called “charming penguins”, a set of processes present in the Standard Model, whose contributions are extremely tricky to predict. Recent estimates of these charming penguins suggest their effects are not large enough to explain our data.
Furthermore, a combination of a theory model and experimental data from LHCb suggests that the charming penguins (and therefore, the Standard Model) struggle to explain the anomalous results.
New data already collected will let us confirm the situation in the coming years: in our current work, we studied approximately 650 billion B meson decays recorded between 2011 and 2018 to find these penguin decays. Since then, the LHCb experiment has recorded three times as many B mesons.
Further advances are planned for the 2030s to exploit future upgrades to the LHC and accrue a dataset 15 times larger again. This ultimate step will allow definitive claims to be made, potentially unlocking a new understanding of how the universe works at the most elementary level.
Reference: “A comprehensive analysis of the 𝐵0→𝐾*0𝜇+𝜇−decay” by LHCb collaboration, 19 December 2025, arXiv.
DOI: 10.48550/arXiv.2512.18053
Adapted from an article originally published in The Conversation.
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